A material estimate has at least three quantities: the amount purchased, the amount usable and the amount required by the job. Keeping those separate prevents a common error: treating a waste percentage as though it always means the same thing.
Calculate usable yield
Suppose a purchased batch contains 100 units and costs 500. After trimming or loss, 80 units are usable.
Yield = 80 ÷ 100 = 80%
Cost per usable unit = 500 ÷ 80 = 6.25
Using 500 ÷ 100 would give a purchase-unit cost of 5, but it would not recover the lost portion through the units available to use.
Why adding 20% is different
If a job needs 80 usable units and the expected yield is 80%, required purchased quantity is:
80 ÷ 0.80 = 100 units
Adding 20% to the 80-unit requirement gives 96. At 80% yield, 96 purchased units produce only 76.8 usable units, leaving 3.2 short.
A “20% waste allowance” can mean either a loss measured against purchases or a percentage added to the net requirement. Ask which convention a supplier, estimator or calculator uses before transferring the number.
Round to the actual pack size
Now assume yield is 90%, the job needs 83 usable units and stock is sold in packs of 25.
Unrounded purchase requirement = 83 ÷ 0.90 = 92.222…
Packs = ceiling(92.222… ÷ 25) = 4
You buy 100 units. Expected usable quantity is 90, leaving seven usable units after the job if the yield assumption holds.
Do not round the purchase requirement down before calculating packs. If only whole usable pieces make sense, also check the physical cutting or grouping constraints: a percentage yield alone cannot describe every layout.
Separate cash paid from cost consumed
At 125 per pack, four packs require 500 of purchasing cash. If each pack yields 22.5 usable units under the model, usable cost is 500 ÷ 90 = 5.5556. Consuming 83 usable units assigns approximately 461.11 to this job, with the remainder associated with leftover material under that convention.
If the leftovers cannot be reused, the full purchase may instead belong in the job estimate. State your convention and follow the applicable accounting treatment for actual inventory records.
The Australian Government’s inventory guidance emphasises keeping track of stock and choosing suitable inventory controls. The yield and pack examples here are independently constructed calculations. Inventory reference.
Use the plumbing pack-order calculator or material yield calculator. Then place the purchase payment in the cash forecast.
About the examples and sources
The figures are hypothetical teaching examples created for AskToBot, not surveyed prices or recommended industry margins. The linked references explain the underlying concepts; they do not supply or endorse these example figures. The arithmetic is checked against the stated assumptions. This is AI-assisted educational content and has not been independently reviewed by an accountant. Read how we check calculations.
Decisions to check next
Sources & references
- Manage your inventory — Australian Government — business.gov.au
